Radio communication system

ABSTRACT

In a radio communication system having a primary station and plurality of secondary stations, power of uplink and downlink channels between the primary station and a secondary station is controlled in a closed loop manner by each station transmitting power control commands to the other station. Responsive to these commands receiving station adjusts its output power in steps. By considering a plurality of received power control commands receiving station may emulate the ability to use power control step sizes other than those it directly implements, for example step sizes smaller than its minimum or intermediate between implemented step sizes. Performance can thereby be improved under certain channel conditions. In one embodiment when required power control step size is less than the minimum step size of the receiving station, that station processes a group of power control commands to determine whether to adjust its output power by its minimum step size.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/575,610, filed May 22,2000, now U.S. Pat. No. 6,556,838.

FIELD OF INVENTION

The present invention relates to a radio communication system andfurther relates to primary and secondary stations for use in such asystem and to a method of operating such a system. While the presentspecification describes a system with particular reference to theemerging Universal Mobile Telecommunication System (UMTS), it is to beunderstood that such techniques are equally applicable to use in othermobile radio systems.

BACKGROUND OF THE INVENTION

There are two basic types of communication required between a BaseStation (BS) and a Mobile Station (MS) in a radio communication system.The first is user traffic, for example speech or packet data. The secondis control information, required to set and monitor various parametersof the transmission channel to enable the BS and the MS to exchange therequired user traffic.

In many communication systems one of the functions of the controlinformation is to enable power control. Power control of signalstransmitted to the BS from a MS is required so that the BS receivessignals from different MS at approximately the same power level, whileminimising the transmission power required by each MS. Power control ofsignals transmitted by the BS to a MS is required so that the MSreceives signals from the BS with a low error rate while minimisingtransmission power, to reduce interference with other cells and radiosystems. In a two-way radio communication system power control may beoperated in a closed or open loop manner. In a closed loop system the MSdetermines the required changes in the power of transmissions from theBS and signals these changes to the BS, and vice versa. In an open loopsystem, which may be used in a TDD system, the MS measures the receivedsignal from the BS and uses this measurement to determine the requiredchanges in the MS transmission power.

An example of a combined time and frequency division multiple accesssystem employing power control is the Global System for Mobilecommunication (GSM), where the transmission power of both BS and MStransmitters is controlled in steps of 2 dB. Similarly, implementationof power control in a system employing spread spectrum Code DivisionMultiple Access (CDMA) techniques is disclosed in U.S. Pat. No.5,056,109.

In considering closed loop power control it can be shown that for anygiven channel conditions there is an optimum power control step sizewhich minimises the required E_(b)/N₀ (energy per bit/noise density).When the channel changes very slowly the optimum step size can be lessthan 1 dB, since such values are sufficient to track changes in thechannel while giving minimal tracking error. As the Doppler frequencyincreases, larger step sizes give better performance, with optimumvalues reaching more than 2 dB. However, as the Doppler frequency isfurther increased there comes a point where the latency (or update rate)of the power control loop becomes too great to track the channelproperly and the optimum step size reduces again, perhaps to less than0.5 dB. This is because the fast channel changes cannot be tracked soall that is needed is the ability to follow shadowing, which istypically a slow process.

Because the optimum power control step size can change dynamically itmay improve performance if the BS determines an appropriate powercontrol step size for use in uplink transmissions from MS to BS anddownlink transmissions from BS to MS, and informs the MS accordingly. Anexample of a system which may use such a method is the UMTS FrequencyDivision Duplex (FDD) standard, where power control is important becauseof the use of CDMA techniques. Although improved performance can beobtained by having a small minimum step size, for example 0.25 dB, thiswill significantly increase the cost of a station. However, if a stationdoes not have to implement the minimum step size then it may not be ableto implement the requested step size.

A further problem may occur in a system in which implementation of somepower control step sizes by a station is optional. For example, in asystem operating according to the UMTS specification a BS may use aplurality of different power control step sizes when changing thedownlink transmission power, for example the four step sizes 0.5 dB, 1dB, 1.5 dB and 2 dB. However, it may be the case that onlyimplementation of a 1 dB step size is mandatory. In some circumstancesit may be desirable to ensure that different BSs behave in a similarway. For example, during soft handover, a MS engages in communicationwith a plurality of BSs (known as the “active set” of BSs) to determineto which BS, if any, it should transfer. It is therefore necessary toavoid the transmission power of the BSs in the active set from divergingsignificantly. This is best achieved if the BSs in the active set changetheir transmission power in similar ways, for example by using similarpower step sizes in response to received power control commands.

If two BSs are in soft handover with a MS which is moving at a speedsuch that 1.5 dB step sizes are optimal, but only one of the BSssupports 1.5 dB steps, optimum power control of both BSs is notpossible. The network has to choose between instructing the BSs to usedifferent step sizes, so that the optimum step size can be used by theBS supporting it (with the risk that the transmit powers of the two BSswill diverge significantly), or instructing both BSs to use the samenon-optimal step size (e.g. 1 dB or 2 dB) in order to avoid unduedivergence of transmit power. Clearly, neither choice is optimal.

SUMMARY OF THE INVENTION

An object of the present invention is to enable selection of optimumpower control step sizes without requiring all stations to implement thesame set of step sizes.

According to a first aspect of the present invention there is provided aradio communication system having a communication channel between aprimary station and a secondary station, one of the primary andsecondary stations (the transmitting station) having means fortransmitting power control commands to the other station (the receivingstation) to instruct it to adjust its output transmission power, whereinthe receiving station has emulation means for emulating an unsupportedpower control step size by a combination of power control steps of atleast one supported size.

According to a second aspect of the present invention there is provideda primary station for use in a radio communication system having acommunication channel between the primary station and a secondarystation, the primary station having means for adjusting its outputtransmission power in steps in response to power control commandstransmitted by the secondary station, wherein emulation means areprovided for emulating an unsupported power control step size by acombination of power control steps of at least one supported size.

According to a third aspect of the present invention there is provided asecondary station for use in a radio communication system having acommunication channel between the secondary station and a primarystation, the secondary station having means for adjusting its outputtransmission power in steps in response to power control commandstransmitted by the primary station, wherein emulation means are providedfor emulating an unsupported power control step size by a combination ofpower control steps of at least one supported size.

According to a fourth aspect of the present invention there is provideda method of operating a radio communication system having acommunication channel between the primary station and a secondarystation, the method comprising one of the primary and secondary stations(the transmitting station) transmitting power control commands to theother station (the receiving station) to instruct it to adjust its powerin steps, wherein the receiving station emulates an unsupported powercontrol step size by a combination of power control steps of at leastone supported size.

The present invention is based upon the recognition, not present in theprior art, that emulation of small power control step sizes by a stationcan provide good performance.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of a radio communication system;

FIG. 2 is a flow chart illustrating a method in accordance with thepresent invention for performing power control in a secondary station;

FIG. 3 is a graph of the received E_(b)/N₀ in dB required for a biterror rate of 0.01 against the power control step size used in dB for aMS moving at 300 km per hour; and

FIG. 4 is a graph of the received E_(b)/N₀ in dB required for a biterror rate of 0.01 against the power control step size used in dB for aMS moving at 1 km per hour.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a radio communication system which can operate in afrequency division duplex or time division duplex mode comprises aprimary station (BS) 100 and a plurality of secondary stations (MS) 110.The BS 100 comprises a microcontroller (μC) 102, transceiver means(Tx/Rx) 104 connected to radio transmission means 106, power controlmeans (PC) 107 for altering the transmitted power level, and connectionmeans 108 for connection to the PSTN or other suitable network. Each MS110 comprises a microcontroller (μC) 112, transceiver means (Tx/Rx) 114connected to radio transmission means 116, and power control means (PC)118 for altering the transmitted power level. Communication from BS 100to MS 110 takes place on a downlink channel 122, while communicationfrom MS 110 to BS 100 takes place on an uplink channel 124.

In a UMTS FDD system data is transmitted in 10 ms frames each having 15time slots. The BS 100 transmits one power control command (consistingof two bits) per slot, where bits 11 (referred to hereinafter forsimplicity as a value of 1) requests the MS 110 to increase its powerand bits 00 (referred to hereinafter as 0) requests the MS 110 todecrease its power. Changes in the required power control step size arenotified separately over a control channel.

In a system according to the present invention this behaviour ismodified when the MS 110 is requested to implement a power control stepsize smaller than the smallest of which it is capable. In this situationthe MS 110 takes no action unless it receives a series of identicalpower control commands, thereby emulating the performance of a MS 110having more precise power control.

Consider for example the case where the requested step size is 0.5 dBand the minimum step size implemented by the MS 110 is 1 dB. The MS 110processes power control commands in pairs and only changes its outputpower if both commands are equal. Hence if the received commands are 11the power is increased, if they are 00 the power is decreased, and ifthey are either 10 or 01 the power is not changed. It may beadvantageous to align the comparison with the transmission of frames,hence to combine the power control commands transmitted in slots 1 and 2of a particular frame, then the commands transmitted in slots 3 and 4,and so on.

Similarly, if the requested step size is 0.25 dB and the minimum stepsize is 1 dB the MS 110 processes power control commands four at a time,and only changes its output power if all four commands are equal. Hencethe power is increased if the received commands are 1111, decreased ifthey are 0000, and unchanged otherwise. Again it may be advantageous toalign the comparison with the frame transmission, combining the commandstransmitted in slots 1 to 4 of a particular frame, then the commandstransmitted in slots 5 to 8 and so on.

Combining the commands received in three or five slots is particularlyadvantageous in the UMTS embodiment being considered because itmaintains alignment with a frame of 15 slots. However, the method is notrestricted to such a system. Consider a general case where the minimumstep size implemented by the MS 110 is S and the step size requested bythe BS 100 is R. In this case the power control commands may be combinedin groups of G, where G=S/R.

FIG. 2 illustrates a method of emulating smaller power control stepsthan the minimum of the MS 110. The method starts, at 202, with the MS110 determining G, the number of commands to be combined in a group andsetting a received power control command counter i to zero. At 204 theMS 110 receives a power control command and increments the counter i.Next, at 206, the value of i is compared with G. If i is less than Gthen the received command is stored and the MS 110 waits to receive thenext command. Otherwise the required number of power control commandshave been received and the MS 110 determines, at 208, if it shouldadjust its power based on the received power control commands. Once thishas been done the counter i is reset to zero (if i is equal to G) or toone (if i is greater than G, which will happen if G is not integer) andthe MS 110 waits to receive the next power control command.

In an alternative embodiment, instead of combining power controlcommands in groups of G the MS 110 keeps a running total of therequested power change and makes a change once the total requested powerchange reaches its minimum step size. For example, if the requested stepsize is 0.25 dB and the minimum step size is 1 dB the sequence ofreceived commands 11010111 would result in the power being increased by1 dB. The MS 110 then subtracts the step actually implemented from therunning total of the requested power change. However, such a scheme ismore complex to implement (since it requires maintaining a running totalof the requested power change) and it appears to provide only a minimalimprovement to the performance of the method.

In a variation of this alternative embodiment, the MS 110 uses a softdecision method in keeping a running total of the requested powerchange, instead of taking a hard decision on each individual powercontrol command. Each power control command is weighted by a function ofthe amplitude of the received signal for that command, as a measure ofthe likelihood of the MS 110 having correctly interpreted the command,before being added to the running sum. For example, the sequence11010111011 might, once weighted, correspond to the sequence ofrequested power changes 0.8 0.3-0.3 0.4-0.1 0.5 0.9 0.8-0.4 0.7 0.5 (inunits of 0.25 dB). This sequence has a running sum of 4.1 which wouldtrigger the MS 110 to execute an upwards step of 1 dB and to reduce therunning sum to 0.1. This variation should provide a slight improvementin the performance of the method.

Two simulations have been carried out to illustrate the effectiveness ofthe method according to the present invention. These examine theperformance of a MS 110 having a minimum step size of 1 dB compared withthat of a MS 110 having a minimum step size of 0.25 dB. The simulationsmake a number of idealising assumptions:

there is a 1 slot delay in the power control loop;

there is no channel coding;

there is perfect channel estimation by the receiver;

equalisation in the receiver is carried out by a perfect RAKE receiver;

no control channel overhead is included in the E_(b)/N₀ figures; thereis a fixed error rate in the transmission of power control commands; and

the channel is modelled as a simple N-path Rayleigh channel.

The first simulation relates to a rapidly changing channel, with a MS110 moving at 300 km per hour in a single path Rayleigh channel with anerror rate for the power control commands of 0.01. FIG. 3 is a graph ofthe received E_(b)/N₀ in dB required for an uplink bit error rate of0.01 against the power control step size used in dB. The solid lineindicates results for a MS 110 having a minimum power control step sizeof 0.25 dB or less, while the dashed line indicates results for a MS 110having a minimum step size of 1 dB which combines power control bits ingroups of two or four to emulate 0.5 dB and 0.25 dB power control stepsizes respectively.

In this situation the best performance is obtained for small step sizesof less than 1 dB. Emulation of 0.25 dB and 0.5 dB steps results in asmall implementation loss of only about 0.05 dB, compared to about 0.6dB if no emulation is performed, demonstrating the usefulness of theemulation method. Increasing the error rate of the power controlcommands to 0.1 produces a general degradation of about 0.2 dB in thereceived E_(b)/N₀, but the performance of the MS 110 with emulated smallsteps remains close to that of the MS 110 with direct implementation ofsmall steps.

The second simulation relates to a slowly changing channel, with a MS110 moving at 1 km per hour in a six path Rayleigh channel with an errorrate for the power control commands of 0.01. FIG. 4 is a graph ofreceived E_(b)/N₀ in dB required for a uplink bit error rate of 0.01against the power control step size used in dB. The lines in the graphare identified in the same way as for FIG. 3.

In this situation there is a small advantage in using power controlsteps of less than 1 dB. As with the first simulation, the resultsobtained using emulated small steps are very close to those with directimplementation of small steps.

In a further application of this method the value of G may be set to avalue other than S/R if it is considered to be advantageous for reasonssuch as reducing the effect of errors in the interpretation of thetransmitted power control commands (for example by averaging over agreater time period). In some circumstances a MS 110 might thereforechoose to use a step size larger than the minimum which it is capable ofimplementing.

A variation of the method described above can be employed for theemulation of unsupported power control step sizes greater than theminimum step size implemented by a station. Consider the case of a BS100 in a system operating according to the UMTS specification. In oneexample of such a system the BS 100 may use one of four step sizes whenadjusting the power of the downlink transmission 122, namely 0.5 dB, 1dB, 1.5 dB and 2 dB, of which only 1 dB is mandatory.

Consider the situation where the BS 100 is instructed by the networkinfrastructure to use 1.5 dB steps but only implements 1 dB and 2 dBsteps. In a method in accordance with the present invention the BS 100considers the received power control commands in pairs. For use duringsoft handover it is advantageous for these groups to be aligned toeither an odd- or even-numbered frame boundary, since a frame includesan odd number (15) of timeslots. The definition of an even or odd framecan be determined from a connection frame number or system frame number.Such alignment ensures that different BSs 100 in the active set, whichare executing an emulation algorithm in accordance with the presentinvention, behave in a similar way.

In the first timeslot of each pair the BS 100 always implements a powerstep of 1 dB in the direction given by the sign of the received powercontrol command, where the sign is considered to be negative if thereceived command is 0 and positive if the received command is 1. In thesecond timeslot, the BS 100 implements a power step of 2 dB if thereceived power control command is of the same sign as that received inthe first slot, or a power step of magnitude 1 dB if the signs areopposite. If a BS 100 only implements 1 dB steps, more than one 1 dBstep could be performed in a single timeslot if a larger step size isrequired by the emulation algorithm. The resultant power changes are:

commands power change 1^(st) slot 2^(nd) slot 1^(st) slot 2^(nd) slot 00 −1dB −2dB 0 1 −1dB +1dB 1 0 +1dB −1dB 1 1 +1dB +2dB

The above method can be generalised to handle the case of emulating stepsizes equal to (x+0.5) dB where the BS 100 can implement steps of x dBand (x+1) dB by having the power step in the first timeslot of x dB andthe power step in the second timeslot of x dB or (x+1)dB as appropriate.

Further generalisation is also possible. Consider the case of emulatingstep sizes equal to (x+a)Δ dB, where Δ is the smallest step sizesupported by the BS 100, x is an integer and 0<a<1. Each time that theBS 100 receives a power control command it performs the followingcalculation:

S _(i) =S _(i−1) +Pa

where P is equal to −1 when the received command has a value of 0 and isequal to +1 when the received command has a value of 1. S_(i−1) isinitialised to zero in the first timeslot, and thereafter is equal tothe value of S_(i) in the previous timeslot.

If S_(i)≦0.5, the size of the power step implemented by the BS 100 is xΔdB. If S_(i)>0.5, the size of the power step implemented by the BS 100is (x+1)Δ dB, and the BS 100 subtracts P from S_(i).

Now consider the case of emulating step sizes equal to (x+a/b)Δ dB,where x, a and b are integers and a<b. The BS 100 considers receivedpower control commands in groups of b. For the soft handover case it ispreferred that the groups are aligned to an odd- or even-numbered frameboundary, for the same reasons as given above for the basic emulationalgorithm.

The BS 100 divides the group of b timeslots into a sub-groups, such thatthere is at most a difference of 1 in the number of timeslots in eachsubgroup. In all timeslots except the last one in each sub-group, the BS100 always implements a power step of magnitude xΔ dB in the directiongiven by the sign of the received power control command in that slot. Inthe last slot of each sub-group, the BS 100 implements a power step ofmagnitude (x+1)Δ dB if the received power control commands in all slotsof that sub-group are of the same sign, otherwise it implements a powerstep of magnitude xΔ dB. This method ensures that the error in powerlevel is never greater than the greater of a/b dB and (1−a/b) dB.

The methods described above may also be further generalised to includethe emulation of any step size intermediate between two step sizessupported by a BS 100 or MS 110.

In the description above, any reference to emulation of step sizes by aMS 110 for controlling the power of the uplink transmission 124 couldequally well be employed by a BS 100 for controlling the power of thedownlink transmission 122, and vice versa.

Further, the detailed description above relates to a system where powercontrol commands are transmitted separately from instructions to astation to set its power control step size. However, the presentinvention is suited for use in a range of other systems. In particular,it can be used in any system in which there is a variable power controlstep size and in which a station is instructed to use a particular valuefor this step. It can also be used in systems in which the power controlstep size is fixed, or at least fixed while a power control step sizeemulation method is being used. The particular step size to be used by astation could be determined by the network infrastructure, the BS 100,or the MS 110. It could also be determined by negotiation between any ofthese entities.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in radio communication systems,and which may be used instead of or in addition to features alreadydescribed herein.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

What is claimed is:
 1. A radio communication system, comprising: aprimary station and a secondary station, at least one of the primarystation and the secondary station configured to transmit power controlcommands to the other station for controlling in a closed loop mannerpower transmitted over a communication channel between the primarystation and the secondary station, the power control commands indicatinga required power control step size that is not directly implemented inthe station receiving the power control commands; and wherein thestation receiving power control commands includes emulation means,responsive to the power control commands, for emulating the requiredpower control step size by using a combination of power control steps ofa size directly implemented in the receiving station.
 2. The radiocommunication system of claim 1, wherein the required power control stepsize is less than the minimum step size directly implemented in thereceiving station.
 3. The radio communication system of claim 2, whereinthe emulation means processes a group of power control commands todetermine whether to adjust the receiving station's output power by theminimum step size directly implemented in the receiving station.
 4. Aprimary station for use in a radio communication system having acommunication channel between the primary station and a secondarystation, the primary station comprising: means for receiving powercontrol commands from the secondary station for controlling in a closedloop manner power transmitted by the primary station over thecommunication channel, the power control commands indicating a powercontrol step size that is not directly implemented in the primarystation; means for adjusting output power of the primary station insteps in response to the power control commands received from thesecondary station; and wherein the means for adjusting includesemulation means for emulating the required power control step size byusing a combination of power control steps of a size directlyimplemented in the primary station.
 5. The primary station of claim 4,wherein the required power control step size is less than the minimumstep size directly implemented in the primary station.
 6. The primarystation of claim 5, wherein the emulation means processes a group ofpower control commands to determine whether to adjust the primarystation's output power by the minimum step size directly implemented inthe primary station.
 7. A secondary station for use in a radiocommunication system having a communication channel between thesecondary station and a primary station, the secondary stationcomprising: means for receiving power control commands from the primarystation for controlling in a closed loop manner power transmitted by thesecondary station over the communication channel, the power controlcommands indicating a power control step size that is not directlyimplemented in the secondary station; means for adjusting output powerof the secondary station in steps in response to power control commandsreceived from the primary station; and wherein the means for adjustingincludes emulation means for emulating the required power control stepsize by using a combination of power control steps of a size directlyimplemented in the secondary station.
 8. The secondary station of claim6, wherein the required power control step size is less than the minimumstep size directly implemented in the secondary station.
 9. Thesecondary station of claim 8, wherein the emulation means processes agroup of power control commands to determine whether to adjust thesecondary station's output power by the minimum step size directlyimplemented in the secondary station.
 10. A method of operating a radiocommunication system having a communication channel between a primarystation and a secondary station, the method comprising: at least one ofthe primary station and the secondary station transmitting power controlcommands to the other station for controlling in a closed loop mannerpower transmitted over the communication channel, the power controlcommands indicating a required power control step size that is notdirectly implemented in the station receiving the power controlcommands; the station receiving the power control commands adjusting itsoutput power in steps in response to receipt of the power controlcommands; and wherein the station receiving the power control commandsadjusts its output power by emulating the required power control stepsize using a combination of power control steps of a size directlyimplemented in the receiving station.
 11. The method of claim 9, whereinthe required power control step size is less than the minimum step sizedirectly implemented in the receiving station.
 12. The method of claim11, wherein the station receiving the power control commands emulatesthe required power control step size by processing a group of powercontrol commands to determine whether to adjust the receiving station'soutput power by the minimum step size directly implemented in thereceiving station.